Is vCloud Director 9.1 supported?
Yes, Zerto supports VCD 9.1 in the 5.5 version series (updates 0-3). Zerto plans to support VCD 9.1 in the to-be-released 6.0u1 and 5.5u4 versions For CSPs, you should consider 5.5u4 as it also provides limited support for the newly released Upgrade Manager for CSPs.
Keep in mind ZVR will not support VCD 9.1 on version 6.0u0 or any other previous versions of ZVR below 5.5u0.
I hope this helps!